1. Field of the Invention
This invention relates generally to soft lithography, and, more particularly, to low thermal distortion molds that can be used in soft lithography.
2. Description of the Related Art
Soft lithography, or stamp lithography, is a technique for transferring a pattern from a master mold to a substrate. In one conventional soft lithography technique, a stamp or mold is fabricated by casting a curable silicone composition, such as a composition including polydimethylsiloxane (PDMS) and an appropriate cross-linker, against a master that includes features representative of a negative of a desired pattern. The curable silicone composition may be cured, peeled away from the master, and then used to form the desired pattern on a substrate. For example, ink may be absorbed into the stamp and then deposited on to a substrate by pressing the inked stamp on to the substrate. For another example, the stamp may be pressed into a liquid resin, which may be cured and separated from the stamp after curing. For yet another example, the stamp may be contacted with a surface and a liquid resin may be infiltrated into the areas between the stamp and the surface. The resin may be cured and the stamp may be released from the cured resin, leaving behind the desired pattern.
Advances in soft lithography have made this technique competitive with other lithographic techniques, such as photolithography. For example, soft lithographic techniques may be used to form features ranging from 0.1μ to 1μ on a silicon substrate. Compared to the lithographic techniques, soft lithographic techniques may also be used to form stamps or molds that have a relatively large area and may include relatively large features. Thus, when compared to traditional photolithography, soft lithographic techniques may offer a relatively low-cost manufacturing option. Moreover, the patterns formed using soft lithographic techniques are typically flexible and therefore soft lithographic techniques may provide the freedom to form patterns on curved surfaces.
The materials that are used to form the soft lithographic molds or stamps typically need to meet a stringent set of requirements. For example, the cured materials used to form the mold or stamp must be able to release from the master mold without damaging or altering the small and intricate features formed in the mold. The cured materials used to form the molds or stamps may also need to be able to form an intimate contact with a surface on which the pattern is to be formed or applied and may also need to adhere to the surface. Moreover, a portion of the mold or stamp may need to be able to release from the pattern formed on the surface at the completion of the pattern transfer process. If an ink is used to form or apply the pattern, the cured material used to form the mold or stamp needs to be able to absorb the ink and then should allow a portion of the ink to be deposited on the surface. One example of a curable material that may meet one or more of these requirements is Sylgard 184, which is a hydrosilylation curable liquid silicone rubber commercially available from the Dow Corning Corporation. Sylgard 184 has been widely used to form molds or stamps for use in soft lithography.
Conventional curable materials used in soft lithography may, however, have a number of disadvantages and/or limitations. For example, polydimethylsiloxane has a relatively low cross-link density, a flexible backbone chain, and relatively weak inter-chain physical interactions. Consequently, cured silicone compositions formed using polydimethylsiloxane may exhibit a relatively low modulus and/or strength, which may lead to collapse of features formed in the mold or stamp when the feature size is below a threshold value. Accordingly, the relatively low modulus and/or strength may limit the feature sizes and/or aspect ratios of features formed in the mold or stamp.
Cured silicone compositions formed using polydimethylsiloxane may also exhibit a relatively high coefficient of thermal expansion. For example, a conventional cured silicone composition formed using polydimethylsiloxane may have a coefficient of thermal expansion about 250 ppm/° C. to about 300 ppm/° C. The thermal expansion may be exaggerated by curing reactions that may cause dimensional shrinkage of the cured material. Temperature variations and the curing shrinkage may induce dimensional changes that may distort features formed in the mold and/or stamp. In some cases, the dimensional changes may make it difficult to register the features formed in the mold or stamp with features formed in or on other layers. This problem may be particularly acute in relatively large area molds or stamps.
Biebuyck (International Publication Number: WO 97/06012) describes a hybrid stamp structure for lithographic processing that may be used to form features having a typical dimension less than 1μ. The hybrid stamp structure includes a deformable layer for accommodating unevenness of the surface of a substrate and/or the stamp structure, a patterned layer in which a lithographic pattern is engraved, and a rigid support layer that may prevent deformation of the stamp under load. However, the three-layer structure of the hybrid stamp presents a number of technical difficulties, which may make this technique economically unfeasible to implement. For example, the rigid support layer is formed of a rigid material such as glass or quartz and is therefore very brittle, non-conformable, expensive, and difficult to handle.
Bruno, et al (U.S. Patent Application Publication No. 2001/0013294) points out a number of other difficulties with the techniques described by Biebuyck. For example, the likelihood that air bubbles may be included in the contact plane, which may lead to zones of failed printing due to missing contact between the stamp and the surface, as well as inhomogeneities in the thickness of the thin-film hybrid stamp due to chemical or physical shrinkage and tolerances in the printing tool that can cause uneven load on different areas of the stamp that lead to collapse of patterns and/or sagging. To address these and other difficulties with previous techniques, Bruno proposes a multi-layered stamp structure that includes a carrier layer made of a metal foil, a thin glass, or a quartz plate having a patterned layer applied to one side and a soft pattern attached to the opposite side. However, the three-layer structure of the stamp described by Bruno still presents a number of technical difficulties that may make this technique economically unfeasible to implement. For example, the carrier layer may be very brittle, non-conformable, expensive, and difficult to handle.